The present invention relates to electro-acoustic transducers comprising at least two piezoelectric polymer diaphragms. More particularly, the present invention relates to an electro-acoustic transducer device which is capable of producing a dipole horn effect.
Piezoelectric films of polyvinylidene flouride resins have been used as electro-acoustic transducers for several years. For example, in U.S. Pat. No. 3,792,204 issued on Feb. 12, 1974, there is disclosed an electro-acoustic transducer having excellent acoustic characteristics which is composed of a piezoelectric film of polyvinylidene flouride resin having an electro-conductive material on the opposite surfaces of the film.
Later research has been directed to obtaining the optimum configuration for electro-acoustic transducers which employ piezoelectric films. One of the first physical configurations developed is disclosed in U.S. Pat. No. 3,947,644 issued on Mar. 30, 1976. In this patent a piezoelectric-type electro-acoustic transducer composed of two convex or concave piezoelectric polymer films is disclosed. Each film has electrodes on both surfaces thereof and the two polymer films are so connected to electric wiring that, when one of the piezoelectric polymer films elongates by the action of an electric field in one direction, the other film shrinks by the action of the same electric field.
Another design for a piezoelectric electro-acoustic transducer is disclosed in U.S. Pat. No. 3,973,150 issued on Aug. 3, 1976. This device employs a uniaxially-stretched film of a shape having the major axis and the minor axis such that the expansion-contraction direction of the film is at a maximum parallel with the minor axis. In this configuration, the transducer can provide a high converting efficiency, especially in the low frequency output range.
A further configuration for piezoelectric transducers is disclosed in U.S. Pat. No. 4,401,911, issued on Aug. 30, 1983. This transducer includes a radiating structure whose active element is formed by a polymer film placed between two electrodes. The transducer includes a closure element having the exact shape of a spherical surface portion and being connected to at least one active peripheral suspension which simulates the movements of a pulsating sphere portion completing the closure element.
In addition, there are several other transducer designs disclosed in U.S. Pat. Nos. 4,186,323 (multi-layer transducers), 4,028,566 (undulated diaphragm) and 4,127,749 (a pair of semi-cylindrical membranes having a single axis of elongation tangent to the curvature).
From the foregoing discussion, it is apparent that several designs for piezoelectric electro-acoustic transducers presently exist. However, none of these designs has been able to achieve a dipole horn effect which provides an acoustical coupling that increases the efficiency of the transducer, or, can provide for a true 360.degree. response. Further, none of the foregoing designs provides a 360.degree. sound dispersion in the vertical plane as well as the horizontal plane.
Thus, there is a need in the art for a piezoelectric electro-acoustic transducer which is capable of producing a 360.degree. response in both the vertical and horizontal planes. Moreover, there is a need in the art for a speaker system which can be centrally located in a room and which will radiate a high efficiency response in all directions to thereby provide accurate sound reproduction to the entire room.